1. Field of the Invention
The present invention relates to a magnetic alloy material that can be used effectively as a magnetic refrigerant material or a magnetostrictive material and also relates to a method of making such a magnetic alloy material.
2. Description of the Related Art
A magnetic alloy, having a composition represented by the general formula: La1-zREz(Fe1-xAx-yTMy)13 (where A is at least one element that is selected from the group consisting of Al, Si, Ga, Ge and Sn; TM is at least one of the transition metal elements; RE is at least one of the rare-earth elements except La; and the mole fractions x, y and z satisfy 0.05≦x≦0.2, 0≦y≦0.1 and 0≦z≦0.1, respectively, and which will be referred to herein as an “LaFe13-based magnetic alloy”) has an NaZn13-type crystal structure and exhibits giant magnetocaloric effect and magnetovolume effect at temperatures around its Curie temperature Tc. Thus, the LaFe13-based magnetic alloy is recently expected to be applicable for use as a magnetic refrigerant material or as a magnetostrictive material (see Japanese Laid-Open Publication No. 2000-54086, Japanese Laid-Open Publication No. 2002-69596 and Asaya Fujita et al., “Huge Magnetovolume Effect and Magnetocaloric Effect of Itinerant Electron Meta-magnetic La(FexSi1-x)13 Compound”, Materia, Vol. 41, No. 4, pp. 269–275, 2002, for example).
In the prior art, the LaFe13-based magnetic alloy is produced by thermally treating a cast alloy, obtained by an arc melting or high frequency melting process, at about 1,050° C. for approximately 168 hours within a vacuum.
The conventional method of making the LaFe13-based magnetic alloy, however, has the following drawbacks.
Specifically, the cast alloy, obtained from a molten alloy having a predetermined composition, has a structure in which at least two crystalline phases with excessively large grain sizes, including an α-Fe phase (as a solid solution of portions of A and TM in the general formula described above) and a phase consisting of the balance, are distributed in a complex pattern as shown in FIG. 6A. A compound phase having the NaZn13-type crystal structure (which will be referred to herein as an “LaFe13-type compound phase”) is produced on the interface between these crystalline phases with excessively large grain sizes as shown in FIG. 6B. Thus, to obtain the LaFe13-based magnetic alloy (as an intermetallic compound) from a structure including such crystalline phases with excessively large grain sizes by the conventional process, the cast alloy should be homogenized by being thermally treated at an elevated temperature for a long time (which will be sometimes referred to herein as a “homogenizing heat treatment”) as described above. The conventional LaFe13-based magnetic alloy cannot be mass-produced sufficiently because such a homogenizing heat treatment must be carried out for a long time to obtain the LaFe13-based magnetic alloy.
In addition, while the cast alloy is processed by the long homogenizing heat treatment, the surface of the alloy may be corroded due to oxidation, thus possibly deteriorating the magnetocaloric effect or magnetovolume effect of the resultant LaFe13-based magnetic alloy.
Furthermore, the cast alloy normally has an ingot shape and is usually subjected to the homogenizing heat treatment as it is (i.e., without being pulverized). However, a magnetic refrigerant material is often used as powder particles to achieve higher heat exchange efficiency with respect to a heat exchange fluid (e.g., a liquid with huge specific heat such as an aqueous antifreeze). Thus, the ingot cast alloy is not so easy to pulverize and may decrease the productivity unintentionally.